During the past years, refractory fibers of heat resistant materials, such as alumina, silica, glasses of various compositions and high melting metals have come into general use for the fabrication of materials for high temperature applications. Refractory fibers have proved their utility as components of thermal insulation and as reinforcing agents in composite light weight materials. Non-metallic fibers, as compared to metals, are more resistant to oxidation and chemical attack. Several processes have been described for the production of refractory non-metallic fibers, two of such processes being disclosed in U.S. Pat. No. 3,269,802 and 3,433,725. Both of these patents describe the treatment of carbon yarn to form carbide fibers; these, however, are rather deficient in tensile strength and are not well suited for applications where the fiber serves as a reinforcing for a composite article. A process or method for strengthening these fibers is therefore desirable, since carbide fibers are well suited in other respects for composite reinforcing media and would find increasing use if the fiber tensile strength could be increased.